This invention relates to actuator positioning systems, and more particularly to an electro-hydraulic system for precisely positioning the output of a hydraulic actuator. In presently preferred embodiments of the invention, the invention relates yet more specifically to electro-hydraulic vehicle power steering systems that incorporate an electric/hydrostatic steering assist module.
Typical power assisted steering systems in use today include a belt-driven high rpm rotary hydraulic pump, specifically engineered hoses, tubes, couplings, an array of brackets and fasteners, and a rack and pinion subassembly. All of these components are engineered to endure the rigors of extreme thermal cycling brought about by a combination of ambient temperatures in the engine compartment, and various operational loads handled by the steering pump under the usual driving conditions. Such power-assisted systems are a source of noise, operating inefficiency and leakage, and consume a large amount of engine power. Power assisted steering pumps are built to very exact tolerances. Many components of these pumps are fabricated under tightly controlled manufacturing processes in order to maintain design specifications. Small discrepancies in manufacturing processes can lead to many performance problems.
A general object of the present invention is to provide an electro-hydraulic system for positioning a hydraulic actuator that obtains improved accuracy at reduced cost. Another object of the present invention is to limit, if not entirely eliminate, many of the problems described above associated with conventional electro-hydraulic actuator positioning systems, including specifically power steering systems. Another object is to provide a power steering system which is composed of a relatively few parts, is rugged and durable in use, and is capable of being inexpensively manufactured and readily installed. A further object of the invention is to provide an electro-hydraulic steering assist module for automotive applications that eliminates any requirement for a conventional rotary-driven power steering fluid pump, which in turn eliminates a major source of fluid-borne noise and increases fuel economy. Another object of the invention is to provide a power steering assist module that employs a rack and pinion assembly that is currently in production, thereby eliminating any necessity for redesigning or requalifying the rack and pinion portion of the steering system.
An electro-hydraulic actuator system in accordance with a presently preferred embodiment of the invention includes a hydraulic actuator having a pair of fluid chambers and being operable to provide an actuator output as a function of fluid pressures at the chambers. An electric-hydrostatic actuator includes an electric motor responsive to motor control signals for providing an output to a motor shaft, one or more pistons coupled to the shaft, and one or more hydraulic cylinders mounted on the motor and cooperating with the pistons for providing a pair of fluid cylinders respectively coupled to the fluid chambers at the hydraulic actuator. A solenoid valve is connected between the chambers, and is responsive to valve control signals for feeding fluid between the actuator chambers and thereby short circuiting the hydrostatic actuator cylinders. A pair of pressure sensors are coupled to the actuator chambers for providing respective pressure signals as a function of fluid pressure at the chambers, and a controller is responsive to the pressure signals for providing the valve and motor control signals. The electro-hydraulic actuator system of the present invention is disclosed in conjunction with electro-hydraulic steering assist modules for automotive steering applications. However, the electro-hydraulic actuator system of the present invention may also be employed in conjunction with marine applications such as rudder and engine tilt controls, aerospace applications such as landing gear, cargo door and flight control surface controls, and industrial applications such as assembly line diverters and horizontal fork positioning controls on fork trucks.
The electric motor in accordance with the preferred embodiments of the invention comprises a rotary motor, and the motor output shaft comprises a leadscrew operatively coupled to the piston or pistons for moving the piston or pistons linearly in associated cylinders. In one disclosed embodiment of the invention, the leadscrew is a rotary leadscrew disposed in axially stationary position, and the piston or pistons are axially movable along the rotary leadscrew. In another disclosed embodiment of the invention, the leadscrew is non-rotatable and has external threads coupled to internal threads on a rotatable armature sleeve disposed within the motor, and a piston is mounted on one or both ends of the leadscrew. Rotation of the armature sleeve translates the leadscrew axially with respect to the motor and reciprocates the piston(s) within the associated cylinder(s). In one embodiment of the invention, a single piston is disposed within the cylinder of a housing mounted to the motor, and the fluid cylinders are formed on opposite sides of the piston. In another embodiment of the invention, housings are secured to the axial ends of the motor, and the leadscrew extends from the axial ends of the motor for coupling to pistons within the associated housings. The housing or housings preferably are formed by an extrusion that has a central chamber within which a piston is disposed, and at least one peripheral chamber that cooperates with passages in end caps for forming fluid passages to and from the fluid cylinders. The control electronics may be mounted in another peripheral chamber, and may include a magnetic sensor for monitoring position of the piston within its associated chamber through the extrusion wall that divides the chambers.
In the disclosed embodiments of the invention having particular utility in electro-hydraulic power steering assist applications, an elongated rack is adapted to be connected at opposite ends to steerable wheels on a vehicle. A rotatable steering gear is in mesh with a series of teeth along a section of the rack, and is adapted to be operatively connected to a steering wheel of a vehicle so as to receive vehicle operator steering input. The rack extends lengthwise within an elongated housing constructed to form a power assist cylinder. A piston is carried by the rack and separates the power assist cylinder into first and second power assist working fluid chambers that are respectively coupled to the fluid cylinders of the electric-hydrostatic actuator. The power assist steering system preferably is entirely self contained, requiring no fluid input from an external rotary pump or the like. The power steering rack and pinion arrangement may be of conventional design, eliminating any requirement for redesigning or requalifying this portion of the steering system. Control parameters such as steering xe2x80x9cfeelxe2x80x9d may be readily configured by software in the control electronics.